1. Field of the Invention
This invention relates to improvements in a valve timing control system for variably controlling the opening and closing timings of intake and/or exhaust valves of an internal combustion engine in accordance with an engine operating condition, more particularly to a device for causing relative rotation between a camshaft to a sprocket which drives the camshaft.
2. Description of the Prior Art
A variety of valve timing control systems of the above-mentioned type have been proposed and put into practical use. Typical one of them is disclosed in the U.S. Pat. No. 4,231,330 and arranged as set forth below. The valve timing control system is arranged to control a camshaft for operating intake and/or exhaust valves of an internal combustion engine. The camshaft is formed at its front end section with an external thread. A sleeve is disposed around the front end section of the camshaft in a manner that its internal thread is engaged with the external thread of the camshaft front end section. A driven sprocket is disposed and supported around the sleeve and the front end section of the camshaft and provided at its outer periphery with teeth to which a rotational force is transmitted through a timing chain from a crankshaft of the engine. The driven sprocket is formed at its inner periphery with an internal thread. Additionally, a cylindrical gear is threadingly disposed between the internal thread of the driven sprocket and the external thread of the camshaft front end section. At least one of the internal and external threads of the cylindrical gear is helical. This cylindrical gear is moved in the axial direction of the camshaft in accordance with an engine operating condition, under the pressure in a hydraulic circuit and the biasing force of a spring, so that the camshaft rotates relative to the driven spocket.
However, in the above-discussed conventional valve timing control system, relative rotation between the driven sprocket and the camshaft is induced using the helical gear which is formed an at least one of the inner or outer peripheral surfaces of the cylindrical gear. This helical gear requires high precision machining to ensure good engagement with the internal thread of the driven sprocket the external thread of the camshaft. Thus, production or (machining) of the helical gear becomes troublesome and difficult, thereby lowering the production efficiency and raising a production cost for the valve timing control system.